Differences in Right Ventricular Remodeling and Regional Function in Primary versus Thromboembolic Pulmonary Hypertension Using Magnetic Resonance Imaging. Pulmonary hypertension (PH) causes adaptive remodeling of the right ventricle (RV), often leading to RV failure. Magnetic resonance imaging (MRI) with tissue tagging (MRTT) provides accurate evaluation of RV structure and regional intramural function. To characterize the ventricular response and remodeling process in primary PH (PPH) vs. thromboembolic PH (TRH) under similar loading, MRI studies were performed on a 1.5 T GE MR scanner using a fast breath-hold sequence and 2D MRTT. Six patients with PPH (mean age 47, 4 female) were compared to seven patients with TPH (mean age 43, 4 female). Pulmonary artery (PA) systolic pressure was similar in PPH and TPH (80+11 v. 75+10 mmHg, p=NS). Short axis images were acquired from the PA to the RV apex using 6 mm slices. Global RV parameters of end diastolic volume (EDV), mass (M), mass-to-volume ratio (M:V), and ejection fraction (EF) were measured by modified Simpson's method. RV wall thickness (RVWT) was measured at the mid-ventricular level. Regional RV function was measured at 3 short axis locations (Basal Inflow, Mid-Ventricle, Apex). Regional 2D Strain parameters E1 (comparable to wall thickening) and E2 (comparable to circumferential shortening) were measured and pooled by location. RV EDV was greater in TPH (110+21 v. 82+17, p<.05), while RVM was similar (p=NS). RV M:V was greater in PPH (1.7 v. 1.3, p<.05), and RVWT was greater in PPH (10+2 v. 7+2, p<.05). RV EF was depressed in both groups, but was greater in PPH (28+3% v. 20+:4%, p<.05). El was similar in the two groups at all locations (p=NS). E2 was greater in the Basal Inflow and Mid-Ventricular regions in PPH v. TPH (-0.125 v. -0.101, p<.05). MRI and MRTT permit characterization of 2 distinct global and regional remodeling patterns in PH, with differing impacts on RV contractile function despite similar loading conditions. These data may provide new insights into the mechanisms and prognosis of RV failure in PH.